1. Field of the Invention
This invention relates generally to a system for monitoring blood pressure, and more specifically, to a combination of devices and a methodology for non-invasively monitoring the blood pressure waveform in a blood vessel by detecting tissue stress in a blood vessel wall and surrounding tissue.
2. Cross-Reference
The following United States patents have common inventorship and common assignee of interest with this application and each of them are hereby incorporated by reference into this specification: U.S. Pat. Nos. 5,158,091; 5,271,405; 5,240,007; 5,195,522; 5,273,046; 5,261,412; and 5,263,584.
3. The Prior Art
Methods for accurately monitoring the blood pressure waveform have been under investigation for some time. While invasive methods can provide accurate waveforms, the trauma caused to a patient makes such techniques undesirable in many cases. One such method involves the use of a fluid filled catheter inserted into a patient's artery. While accurate blood pressure measurements can be obtained by this method, the negative effects on the patient often outweigh the benefits of achieving accurate results from such a method.
Routine methods of monitoring a patient's blood pressure waveform include the widely used ausculatory method known as the Korotkoff method. This method is non-invasive, however, it only provides a measurement of systolic and diastolic pressure on an intermittent basis; it does not provide the entire waveform on a continuous basis. Furthermore, use of the Korotkoff method often yields inaccurate results. Moreover, the rate at which blood pressure can be recorded is limited by the inflation and deflation rate of the occlusive cuff. Therefore, true beat-to-beat continuous blood pressure monitoring is not possible using this method.
While the occlusive cuff instruments have been adequate for ascertaining long term trends in patient blood pressure, short term variation has previously not been easily measured non-invasively. Techniques that offer potential in this area include a method using a pressure-feedback technique that records the blood pressure in a patient's finger. Feedback error signals are obtained using optical plethysmography. Such methods include several draw backs. One is that blood pressure measurement is too peripheral and undesirably influenced by the smooth muscle tone of the artery. Secondly, it is difficult to implement arterial tonometry with previously available devices because a high degree of miniaturization is required for contact stress sensors used in available devices. For example, one type of arterial tonometer includes an array of individual transducer elements placed directly on the patient's tissue overlying an artery or blood vessel from which blood pressure is to be determined. The elements directly sense the mechanical forces in the tissue with which each of them is in contact. The elements of the array are dimensioned and spaced apart from each other such that a plurality of these elements are required to cover the entire diameter or width of the underlying blood vessel; the size of each element is assigned to cover only a small fraction of the diameter of the underlying blood vessel. The pressure of the array against the tissue is increased to properly applanate the underlying vessel without causing occlusion. The fluid pressure within the artery is then conducted through the vessel wall and the overlying tissue to the transducers.
A significant draw back to such devices includes the use of the discrete elements. It is believed that with such tonometers a continuous contour of the tissue stresses under the array is not accurately obtained because the discrete elements inherently confine the system resolution. Additionally, it is believed that in prior methods no compensation means is provided for motion artifacts which may affect the forces translated to the sensors from the artery.
In view of the above, there is a need for true beat-to-beat, continuous arterial blood pressure measurement. Current research indicates that changes in the pulse waveform due to wave reflection can be responsible for an increase in systolic pressure. Monitoring such a pulse waveform can be crucial, for example, during surgical procedures. Cuff-based techniques are used to monitor blood pressure during surgery. However, a cuff-based technique provides limited ability to monitor the pulse waveform on a continuous basis. Similarly, continuous measurement of pressure during exercise has been limited.
Therefore, it is desirable to provide an arterial tonometry system that is designed for noninvasively continuously measuring arterial blood pressure. This invention addresses these needs and provides the additional capability of determining short term or long term blood pressure trends for any particular patient, depending on the needs of a particular situation.